What is aid for? We've been offered two different answers to that question by the UK government in the last couple of weeks. First off was the Department for International Development's private sector strategy, in which we were told that aid was about generating "opportunity and prosperity", for poor people through promoting economic growth (pdf) and the private sector.

Then we've seen the very high-profile conference on vaccines, where David Cameron promised that British aid would "save millions of lives".

So which is it? Aid as an investment in growth in the future, or aid as a form of welfare to save lives now? The new focus on results and value for money at DfID brings into sharp focus the difference between these two approaches, in particular the very different roles that aid plays in promoting growth or in saving lives.

Aid for economic growth and aid for vaccines bring about change in very different ways. A vaccine put into any arm, anywhere in the world, does the same thing. But how economies grow, and what impact that has on poor people, is all about specific and complicated relationships that depend on the country-, province- or even village-level circumstances.

This makes it more complicated for aid donors. While it's possible to estimate that for a given amount of money you can vaccinate a certain number of kids, it's much harder to design a credible "money in, results out" formula for aid that promotes growth.

Monday's pledge from the UK government on vaccines is apparently going to vaccinate 80 million children and save 1.4m lives. The private sector strategy, by contrast, promises to provide 40 million people with access to financial services, and to mobilise "pro-poor investment".

One is rather less specific than the other, in terms of the concrete difference that this aid is expected to make to people's lives. If you provide a vaccine, you can be reasonably sure that the child vaccinated won't get sick. But if you provide someone with access to financial services, you can't know if they will use any credit for borrowing or for investment, or if in fact they will borrow too much, have problems paying back and end up worse off. But that's the dilemma donors face when they get involved in messy and unpredictable processes like economic growth.

So what to do? One plausible reading of the evidence would be to say that although economic growth is key to development, it's not the business of aid. Aid isn't really that good at promoting "opportunity and prosperity" – the evidence base is too weak, the results too uncertain, and the whole thing so contingent on everything else that it really wouldn't be a good use of taxpayers' money. On this reading, aid should be about things that we really know work and are easy to monitor – such as, for example, vaccines.

A second way would be to say that given how important economic growth is to development as a whole, it would be ridiculous not to use aid to give that process a push in the right direction for poor people. Ultimately, the gains from getting growth right would mean that a country could pay for its own vaccines and lots more – the prize is so great that it's worth the extra risk.

Whatever side you come down on, what's certain is that thinking harder and being more explicit about what results UK aid is designed to achieve has made some of these difficult decisions more open and public than they used to be. Forcing people who make decisions about aid to say what they're getting for the money means the impact of spending on economic growth and vaccines, and all the other things that make up the aid budget, can be compared directly in a way that used to be much harder, and the trade-offs are more visible.

More information about results can't answer the question about whether aid should be spent on vaccines or economic growth. In the end, that's a political judgment. But information can show what the impact of those decisions is likely to be, and help the rest of us to judge whether the choices made are good ones.